In automotive vehicles, fuel vapor may be generated in a fuel tank during engine operation, over diurnal cycles, and during refueling events. Vehicles sold in North America are required to utilize a carbon canister to collect vaporized fuel from the fuel tank, in order to reduce the quantity of fuel vapors released to the atmosphere. The vapors stored in the canister may then be purged from the canister into the engine intake manifold for combustion. In this way, fuel vapors may be recycled to the engine rather than leaked to the environment.
In many examples, pressure differentials within the engine may be utilized to draw fuel vapors from the canister into the intake manifold. For example, engine intake vacuum may be applied to the canister, thus drawing atmospheric air through the canister and into the engine intake. However, in boosted engines, intake manifold pressure may vary substantially depending on whether the compressor is operating. In non-boost conditions, when the compressor is not operating, the intake manifold may have a negative pressure. In contrast, during boost conditions when the compressor is operating, the intake manifold may have a positive pressure. Canister purging in boosted engines must be enabled during both vacuum conditions and boost conditions.
Other attempts to address canister purging in boosted engines include using a venturi effect to generate a vacuum using a positive pressure source. One example approach is shown by Kempf et al. in U.S. Pat. No. 9,109,550. Therein, an ejector or venturi is used as a vacuum source in a dual path purging system. An inlet of an ejector may be coupled to an engine intake upstream of a compressor via a first conduit and an outlet of the ejector may be coupled to an intake of the engine downstream of the compressor via a second conduit. Motive fluid through the ejector provides a vacuum at an ejector suction inlet which is coupled to the fuel vapor canister to draw purge air through the fuel vapor canister during boosted operation.
However, the inventors herein have recognized potential issues with such systems. As one example, the purge path for boost conditions is considerably longer than that for non-boost conditions, as the fuel vapor must pass through the intake air compressor and charge air cooler before reaching engine intake. The increased path length results in a hydrocarbon transport delay, which increases the risk of engine hesitation during purge events. Additionally, the amount of vacuum that can be generated by recirculation flow through an ejector is limited by the ejector choke flow, resulting in a limited amount of fresh air flow through the canister. Further, in many engine conditions, the intake manifold has neither enough pressure nor vacuum to generate purge air flow via either purge pathway.
In one example, the issues described above may be addressed by a method for an engine, wherein during a first condition, pressurized gas from an engine coolant degas bottle to an ejector positioned in a vent line coupled to a fuel vapor canister; and the contents of the fuel vapor canister are purged to an engine intake. The ejector may draw atmospheric air into the fuel vapor canister, thus enabling purging of the fuel vapor canister even when an engine intake vacuum is below a threshold. In this way, boosted engines and other engines configured to operate with reduced intake vacuum may execute canister purging events that are independent of engine intake pressure.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.